This invention relates to drive systems for mini-loader or like machines which incorporate hydraulically driven wheels or sprocket/tracks as well as hydraulically operated functional mechanisms. While the present description refers to a drive system for a loader or mini-loader machine, it will be understood that the present invention is not restricted to this type of unit. The invention is applicable to units that incorporate tracks instead of wheels; units that could be a backhoe or other type of machine, any of which would benefit from the drive system of the present invention.
There are several manufacturers producing compact earth moving machines such as small loaders. Typical examples are now made by Leon""s Manufacturing Company, Inc.; Toro and Kanga. A typical, conventional arrangement is illustrated and described in Canadian Patent 1,236,053 of May 3, 1988.
With minor variations, these machines all use a common basic drive system in which a single hydraulic pump, usually driven by a constant speed gasoline engine, is connected to a control valve that provides control for all drive hydraulic motors, loader cylinders, as well as auxiliary functions. A recent innovation in the art has been to provide a second hydraulic pump for auxiliary functions but the drives are operated by a common control valve and pump.
In order for this convention arrangement to operate properly, the control valve must be a xe2x80x9cseriesxe2x80x9d valve. When more than one lever is operated on a normal control valve, hydraulic oil is allowed to flow to more than one function. However, the oil will follow the path of least resistance and only the function which requires the lowest pressure at the time will actually operate. Accordingly, if such a valve is used in a basic drive system for example, it would not be possible to drive the unit in a straight line or lift the load while moving. Thus, the unit would be uncontrollable.
Series valves solve this problem by forcing the oil coming xe2x80x9cbackxe2x80x9d to the valve from one function to go xe2x80x9coutxe2x80x9d to the next function when a second lever is pulled. Thus, several functions can be operated simultaneously. For example, the machine can be driven properly as the oil coming out of one wheel motor is going into the other one.
However, one main disadvantage of this conventional system using a series control valve is that the pressures are additive because any functions will see any other functions downstream thereof as xe2x80x9cback pressurexe2x80x9d. Accordingly, if the machine was being driven in a straight line and the pressure required to turn each wheel motor was 1000 psi, the total pressure required would be 2000 psi. If an operator tried to lift a load at the same time and the load required 1500 psi to lift, the total system pressure would become 3000 psi.
Generally speaking, the hydraulic systems in these units operate at a maximum pressure of approximately 3000 psi and when this pressure is reached, the system relief valve opens, dumping all the oil back to the reservoir, and everything stops until the pressure drops below the limit again. The result of this is that an operator would either have to stop driving to lift the load or stop lifting the load in order to drive. This greatly limits the operation of the unit as it is fairly common to reach this pressure while driving only, such as turning in loose soil or digging a hole for example. In effect, one can only have half of the power available which the unit is capable of generating. This becomes particularly apparent when operating an attachment from the auxiliary outlets that requires a lot of power and that the unit has to be moving to use. A trencher, snowblower, or tiller would be examples of this type of application.
The present invention addresses the above mentioned disadvantages and weaknesses of the current arrangement for drive systems and provides a unique solution to these short comings. In the system according to the invention, a tandem hydraulic pump is used which in effect is two separate pumps in a common body. However, two interconnected individual pumps could also be used. Each of these pumps is connected to a multiple spool series valve, preferably at least a three spool series control valve. This creates two separate hydraulic circuits, each of which has a separate relief valve.
In the illustrated example described hereafter, each of the multiple, at least, three spool valves controls one wheel motor and one loader function such as the lift or a bucket and a third spool on each valve is a single acting spool and these two xe2x80x9cthirdxe2x80x9d spools both connect to a separate spool valve to provide auxiliary power control.
In this arrangement, wheel motors are in separate hydraulic circuits and operate independent of each other. Thus one wheel motor does not see the other one as back pressure and each has the full flow and pressure of its own pump available to it. This dramatically improves the driving ability of the unit as it results in double the power and torque available at the wheel motors than conventional versions, all else being equal.
According a broad aspect, the invention relates to a drive and operating system for mini-loader or like machines incorporating hydraulically operated wheels and other functional mechanisms. The drive and operating system comprises a pair of hydraulic pumps and engine means for driving the pumps; a pair of multiple-spool series control valves, each hydraulically connected to one of the hydraulic pumps to provide two separate hydraulic circuits; a driving wheel and a loader function each being hydraulically and separately connected to individual spools in each of the multiple-spool series control valves; and a separate spool control valve hydraulically connected to a third spool in each of the multiple-spool series control valves, the separate spool control valve providing power central to an auxiliary function.